1. Field of the Invention
The present invention relates to transformers and electrical devices using the same. In particular, the present invention relates to a transformer to be used for a switching power supply device and to an electrical device using the transformer.
2. Description of the Related Art
FIG. 10 is an exploded perspective view of a known thin transformer. In FIG. 10, a transformer 1 includes flat coils 2, 3, and 4 which are formed by winding wires in spirals and which coaxially overlap each other with doughnut-shaped insulative sheets 5 and 6 therebetween, and core members 7 and 8 sandwiching the flat coils 2, 3, and 4 and the insulative sheets 5 and 6. The flat coils 2, 3, and 4 coaxially overlapping each other are individually provided with holes for passing a magnetic core-leg formed in central parts of the flat coils 2, 3, and 4. The core members 7 and 8 are each provided with a magnetic core-leg.
FIG. 11A is a plan view of the flat coil 3 of the transformer 1. FIG. 11B is a sectional view along line Axe2x80x94A of the flat coil 3 shown in FIG. 11A. In FIG. 11A, the flat coil 3 is formed with a wire 3a wound in a spiral. An inner end 3b of the wire 3a is drawn to the outside over the other part of the wire 3a. An outer end 3c of the wire 3c is drawn to the outside in the same winding direction.
The thickness of the overall flat coil 3 thus formed is substantially the same as the diameter of the wire 3a. However, the thickness of the flat coil 3 is at least twice the diameter of the wire 3a in a portion of the flat coil 3 over which the inner end 3b of the wire 3a is drawn to the outside. The flat coils 2 and 4 each have the same configuration as the flat coil 3 shown in FIGS. 11A and 11B.
The transformer I shown in FIG. 10 includes the flat coils 2, 3, and 4 overlapping each other, each having the thickness twice the diameter of the wire 3a, whereby the thickness of the flat coils 2, 3, and 4 becomes six times the diameter of the wire 3a. Since the transformer 1 also includes the insulative sheets 5 and 6 each having a given thickness, there is a problem in that the thickness of the transformer 1 is increased.
Accordingly, it is an object of the present invention to provide a transformer which can be reduced in thickness and an electrical device using the transformer.
To the end, according to an aspect of the present invention, a transformer comprises a plurality of flat coils overlapping each other, each formed by winding a wire in a spiral. Respective inner ends of the wires forming at least two of the plurality of flat coils are drawn out through a hole for passing a magnetic core-leg and over one of the plurality of flat coils. The respective inner ends are disposed on the same surface of the one of the plurality of flat coils.
The inner ends of the other two of the plurality of flat coils may be connected to each other.
In the transformer according to the present invention, respective outer ends of the wires forming two of the plurality of flat coils may be connected to each other.
The wire forming at least one of the plurality of flat coils may be a three-layer insulated wire.
The wire may be a self-welding-type three-layer insulted wire.
An electrical device according to the present invention is provided which comprises the transformer described above.
The transformer according to the present invention can be reduced in thickness by arranging the same as described above.
The electrical device according to the present invention can be reduced in thickness and in size.